The most widely accepted therapy for treating respiratory infections caused by Gram-negative bacteria in cystic fibrosis patients involves intravenous administration of a single antibiotic or combinations of antibiotics (Gibson et al., 2003; Ramsey, 1996). This method of treatment has several significant limitations including: (1) narrow spectrum of activity of existing antibiotics, (2) insufficient concentrations of antibiotic reaching the respiratory tract to ensure rapid onset and high rates of bacterial killing, and (3) development of adverse side affects due to high systemic concentrations of drug.
Aerosol administration of antibiotics (Conway, 2005; O'Riordan, 2000) addresses several of the limitations of parenteral administration (Flume and Klepser, 2002; Kuhn, 2001). It enables topical delivery of high concentrations of drug to the endobronchial spaces and reduces side effects by lowering systemic exposure to antibiotic. However, cystic fibrosis patients typically receive prolonged and repeated antibiotic therapies over the entire duration of their adult lives (Gibson et al., 2003; Ramsey, 1996). Therefore, cummulative aminoglycoside toxicity and development of resistance remains a significant problem.
Fosfomycin
Fosfomycin is a broad spectrum phosphonic acid antibiotic (Kahan et. al., 1974; Woodruff et al., 1977) that has bactericidal activity against Gram-negative bacteria including Citrobacter spp., E. coli, Enterobacter spp., K. pneumoniae, P. aeruginosa, Salmonella spp., Shigella spp., and S. marcescens (Greenwood et. al., 1992; Grimm, 1979; Marchese et. al., 2003; Schulin, 2002), as well as Gram-positive bacteria including vancomycin resistant enterococci, methicillin-resistant S. aureus (MRSA), methicillin-sensitive S. aureus (MSSA), and S. pneumoniae (Greenwood et. al., 1992; Grimm, 1979; Perri et. al., 2002). Fosfomycin has the greatest activity against E. coli, Proteus spp., Salmonella spp., Shigella spp., and S. marcescens which are generally inhibited at fosfomycin concentrations: 564 μg/mL (Forsgren and Walder, 1983). Fosfomycin is moderately active against P. aeruginosa (Forsgren and Walder, 1983), particularly when compared to tobramycin (Schulin, 2002).
Fosfomycin is bactericidal but exhibits time-dependent killing against E. coli and S. aureus (Grif et al., 2001). The rate and degree of killing depends on the length of time fosfomycin is in contact with the target organism (Craig, 1998; Mueller et al., 2004). Increasing the fosfomycin concentration will not produce a corresponding increase in the rate or degree of killing activity. This feature is significant because it is preferable to treat P. aeruginosa infections with antibiotics that exhibit bactericidal, concentration-dependent killing activity (Craig, 1998; Mueller et. al., 2004).
Fosfomycin monotherapy is commonly used in the treatment of uncomplicated urinary tract infections caused by E. coli, and less frequently in treating bacterial respiratory infections including in patients with cystic fibrosis (Kamijyo et al., 2001; Katznelson et al., 1984; Kondo et al., 1996; Reeves, 1994; Bacardi et al., 1977; Bonora et al., 1977; Honorato et al., 1977; Menendez et al., 1977). However, fosfomycin alone has not been widely used to treat infections caused by P. aeruginosa. Fosfomycin has been administered parenterally in combination with antibiotics of different classes to treat endocarditis (Moreno, et. al., 1986) and cystic fibrosis (Mirakhur et. al., 2003) but has not been delivered directly to the lung environment by aerosol administration.
Fosfomycin is available in both oral (fosfomycin calcium and fosfomycin trometamol) and intravenous (fosfomycin disodium) formulations (Woodruff et al., 1977). Fosfomycin trometamol is the preferred formulation for oral administration because it is more readily absorbed into the blood compared to fosfomycin calcium. Following a single intravenous or intramuscular dose of 2 g of fosfomycin, peak serum concentrations range between 25-95 μg/mL within 1-2 hours (Woodruff et al., 1977). By comparison, concentrations reach 1-13 μg/mL in normal lung after parenteral administration of a comparable dose of fosfomycin (Bonora et al., 1977) which is an insufficient to kill most bacterial pathogens, in particular P. aeruginosa (Forsgren and Walder, 1983; Schulin, 2002). Patients with cystic fibrosis have altered pharmacokinetics characterized by an increased volume of distribution and rate of clearance (Tan et al., 2003), which would likely further decrease the efficiacy of parenterally administered fosfomycin.
Fosfomycin is widely distributed in various body tissues and fluids but does not significantly bind to plasma proteins (Mirakhur et al., 2003). Consequently, fosfomycin is available to exert antibacterial effects if it reaches sufficient concentrations at the site of infection. The respiratory tract of cystic fibrosis patients are obstructed with viscous secretions called sputum (Ramsey, 1996). The effectiveness of several classes of antibiotics such as aminoglycosides and β-lactams is reduced due to poor penetration into sputum. Additionally, the activity of these antibiotics is further reduced by binding to sputum components (Hunt et al., 1995; Kuhn, 2001; Ramphal et al., 1988; Mendelman et al., 1985).
Development of resistance in bacteria isolated from patients treated with fosfomycin for urinary tract infections occurs very infrequently (Marchese et al., 2003). Cross-resistance with other classes of cell wall inhibiting antibiotics does not occur because fosfomycin acts on the enzyme phosphoenolpyruvate (UDP-N-acetylglucosamine enolpyruval-transferase) which is not targeted by other antibiotics (Kahan et al., 1974; Woodruff et al., 1977). Fosfomycin is actively taken up into bacterial cells by two transport systems; a constitutively functional L-α-glycerophosphate transport and the hexose-phosphate uptake system (Kahan et al., 1974). When fosfomycin resistance occurs, it is typically due to a genetic mutation in one or both of the chromosomally encoded transport systems, and less commonly by modifying enzymes (Area et al., 1997; Nilsson et al., 2003).
Tobramycin.
Tobramycin is an aminoglycoside antibiotic that is active against Gram-negative aerobic bacilli including P. aeruginosa, E. coli, Acinetobacter spp., Citrobacter spp., Enterobacter spp., K. pneumoniae, Proteus spp., Salmonella spp., S. marcescens, and Shigella spp (Vakulenko and Mobashery, 2003). In particular, tobramycin is highly active against P. aeruginosa. The tobramycin MICs of susceptible P. aeruginosa are typically less than 2 μg/mL (Shawar et al., 1999; Spencker et al., 2002; Van Eldere, 2003). Most Gram-positive bacteria are resistant to tobramycin, with the exception of S. aureus and S. epidermidis (Vakulenko and Mobashery, 2003).
Tobramycin is rapidly bactericidal and exhibits concentration-dependent killing (Vakulenko and Mobashery, 2003). Increasing the tobramycin concentration increases both the rate and extent of bacterial killing. Therefore, to achieve therapeutic success, it is necessary to administer a large enough dose to produce a peak tobramycin level 5-10 times greater than the MIC of the target organism at the site of infection. It is preferable to treat P. aeruginosa infections with antibiotics that exhibit bactericidal, concentration-dependent killing activity (Ansorg et al., 1990).
Tobramycin is usually administered to treat less serious Gram-negative bacterial infections (Vakulenko and Mobashery, 2003). However, it may be combined with other classes of antibiotics to treat severe infections of the urinary tract and abdomen, as well as endocarditis and bacteremia (Vakulenko and Mobashery, 2003). Parenteral administration of tobramycin in combination with cell-wall inhibiting antibiotics has been used to treat respiratory infections, in particular those caused by P. aeruginosa in CF patients (Gibson et al., 2003; Lang et al., 2000; Ramsey et al., 1999; Ramsey et al., 1993; Smith et al., 1999; Spencker et al., 2003).
Tobramycin is poorly absorbed orally and must be administered parenterally (Hammett-Stabler and Johns, 1998). Tobramycin is available in both intravenous and aerosol formulations. After parenteral administration, tobramycin is primarily distributed within the extracellular fluid. Tobramycin is rapidly excreted by glomular filtration resulting in a plasma half-life of 1-2 hours (Tan et al., 2003). Penetration of tobramycin into respiratory secretions is very poor and its activity is further reduced by binding to sputum (Kuhn, 2001). Aerosol administration of tobramycin results in significantly higher sputum levels of ≧1000 μg/mL (Geller et al., 2002) compared with parenteral administration, but sputum binding remains a significant problem (Hunt et al., 1995; Mendelman et al., 1985; Ramphal et al., 1988).
Nephrotoxicity and ototoxicity are adverse reactions associated with tobramycin therapy (Al-Aloui et al., 2005; Hammett-Stabler and Johns, 1998). Nephrotoxicity results from accumulation of tobramycin within lysosomes of epithelial cells lining the proximal tubules. This causes an alteration of cell function and ultimately cell necrosis (Mingeot-Leclercq and Tulkens, 1999). Clinically, this presents as nonoliguric renal failure. The prevalence of nephrotoxicity in cystic fibrosis patients is estimated to be 31-42% (Al-Aloui et al., 2005). The incidence of ototoxicity, which is characterized by loss of hearing and dizziness, is estimated to be as high as 25% of patients treated with aminoglycosides (Hammett-Stabler and Johns, 1998). Unlike nephrotoxicity, ototoxicity is irreversible. The greatest risk factor for the development of toxicity is cumulative exposure to large doses of tobramycin (Hammett-Stabler and Johns, 1998; Mingeot-Leclercq and Tulkens, 1999). Cystic fibrosis patients are treated with prolonged and repeated high-dosages of tobramycin over their entire lifetime (Tan et al., 2003) and are at increased risk of developing cumulative renal failure (Al-Aloui et al., 2005).
Bacterial resistance to tobramycin has become increasingly prevalent and is due to repeated and prolonged antibiotic monotherapy (Conway et al., 2003; Van Eldere, 2003; Mirakhur et al., 2003; Pitt et al., 2005; Schulin, 2002). For example, Cystic fibrosis patients are colonized with P. aeruginosa strains which are largely resistant to tobramycin gentamicin, ceftazidinme, piperacillin, and ciprofloxacin (Eldere, 2003; Pitt et al., 2005; Pitt et al., 2003; Weiss and Lapointe, 1995). Thus, existing antibiotic therapies are becoming ineffective for treating P. aeruginosa infections because of drug resistance.
It is clear that there is a continued need for an improved method of treatment for acute and chronic respiratory infections caused by Gram-negative and Gram-positive bacteria, particularly multidrug resistant P. aeruginosa. This is particularly evident in cystic fibrosis patients where current therapies are limited by problems with development of resistance and toxicity. Such method of treatment would preferably comprise inhalation of an aerosolized antibiotic combination of fosfomycin and an aminoglycoside such as tobramycin that delivers a therapeutically effective amount of the drugs directly to the endobronchial space of the airways or to the nasal passages. Such treatment would be efficacious, reduce the frequency of drug resistance, and improve safety.
It would be highly advantageous to provide a formulation and system for delivery of a sufficient dose of fosfomycin plus an aminoglycoside such as tobramycin in a concentrated form, containing the smallest possible volume of solution or weight of dry powder which can be aerosolized and delivered predominantly to the endobronchial space.
Thus, it is an objective of this invention to provide a concentrated liquid or dry powder formulation of fosfomycin plus aminoglycoside which contains sufficient but not excessive amounts of fosfomycin and aminoglycoside which can be efficiently aerosolized by nebulization into aerosol particles sizes predominantly within a range of 1 to 5 um and having salinity that is adjusted to permit generation of a fosfomycin plus aminoglycoside aerosol well tolerated by patients, and which has an adequate shelf live.